snowman51789

I have a 257 mirage with a 98 454 mag MPI GEN 6. it runs great except for at idle. when its been idling the motor acts like it is loading up on fuel. it will run good at idle for about 3 mins or so then you can here the engine start sound like its having a hard time keeping 600 rpm it will not stall its self out when just idleing but it has stalled a few times when I was shifting from forward to reverse while I was docking. you can also smell fuel when standing behind the boat on the dock. when you go to get on plane the engine will stutter a little bit but it dose clear its self out and runs great on plane. another issue I noticed is that if the engine is hot and you shut it off and let it sit for 15 mins, it wont start back up unless you give it 1/2 throttle, cold starting is good, and starting it right after you shut it off when it was hot is also normal.

things I checked

1. checked all plugs, #4 and 5 were wet with fuel but all others looked good.
2. TPS voltage, it was reading .85v at idle so I re-adjusted it to .55v at idle
3. cap and rotor look good.
4. Map sensor was reading
5. water temp sensor was also reading accurate (replaced last year)
6. IAC was functioning, was about 40 at idle
7. No water in the oil
8. Fuel pressure is about 37 pounds, when I shut the boat off it drops to 33 or so and did not drop for over 30 mins

I am out of ideas of what could be causing this, any help would be appreciated, thanks

snowman51789

These are the readings I was getting when the engine was warm.

snowman51789

Engine running





Engine off for about 25 mins or so

SUPERSTITIOUS

AllDodge

iTrader: (1)

Is the timing changing a lot or fairly steady?
Have you checked compression?

With engine idling, pull plug wires one at a time to see if engine stumbles with each plug
Get a noid light and see if each injector is firing

snowman51789

The compression readings were between 150 and 160, (that made me happy). The timing numbers are constantly changing but not by very much, I believe that's because the 454 mag mpi some how use timing to keep idle rpm (from what I've heard).

the injectors were flow tested and matched 2 years ago when I had the heads rebuilt.

The boat runs normal when it's not idling. It will run 60 with 2 ppl and a full tank. The fastest I've ever had it is 62. I've had it for 5 years.

Trash

This is a classic case of the VST fuel pump diaphragm going bad. Check the yellow fuel vent lines off the VST pump assembly. They are likely percolating fuel into the TB or manifold causing your idle / hot start issue. Basically you are flooding out the engine. The VST assembly or likely needs a new diaphragm kit installed.

SB

iTrader: (1)

^^^Good advice if it has VST. I assumed (bad to assume right) that a 98 GEN VI is cool fuel.

snowman51789

It is cool fuel. When I was researching, I found some posts describing symptoms of of a bad VST (I didn't know what a VST was) I got real excited. Then i researched what a vst was and realized I don't have one since I have a cool fuel system, then my heart sank. Lol

snowman51789

I did not try unplugging the spark plugs to see if it stumbles but I did plug a spark plug light in-line and all checked out good.

a few months ago when this issue first started, my neighbor plugged in a tool that tests the injectors, its a higher tech version of a noid light from what I understand and all tested good. I never herd of noid lights before you mentioned them. I will try to get a set a check them again.

I bought all new spark plugs, cap, and rotor. I'll try changing them and see what happens, I'm not very hopeful though because the current ones only have 1 season on them.

I was once told that spark plug wires are replaced way to often and that 99% of the time, if they are not visually dry rotted, cracking, or damaged then they are fine. The guy also stated that spark plug wires don't "wear out" (besides dry rot) and that they either work or don't work. Has anybody ever had any other first hand experience with this? Or can anybody confirm this advice?

Thanks